Affinity maturation of the immune response is a process unique to the immune system. This process consists of somatic hypermutation of the variable domains of immunoglobulins, which leads to a significant increase in the affinities of the antibodies elicited during a humoral immune response. The understanding of the molecular and structural basis of this phenomenon is an area of particular interest for Immunology as well as the biological sciences in general. Several authors have described, in detail, models of affinity maturation in antibody-hapten systems. However, affinity maturation against protein antigens has not been described in such detail. Our previous work, as in other laboratories, has shown that the antibodies elicited during the immune response against a protein antigen show a fast maturation of the affinity, a phenomenon closely related to the early IgM-IgG isotype class switch process. No structural, sequence or functional studies describing the changes that occur at the combining site of anti-protein antibodies during this process can be found in the current literature. The aim of this project is to characterize the sequence and the binding site structures of the IgM unmutated precursors of the structurally well characterized IgG anti-hen egg-white lysozyme (HEL) antibodies. This goal will be accomplished by cloning and sequencing anti-HEL IgMs and by X-ray crystallographic structure determinations of the Fv fragments of these IgMs. Structural, functional and mutational studies of the recombinant Fv fragments of those IgMs would reveal the nature of the changes produced during affinity maturation.